Apparatus for pulling rod-shaped crystals of semiconductor material from a melt in acrucible



July 5 1966 'r. RUMMEL 2 APPARATUS FOR PULLING ROD-SHAPED CRYSTALS OFSEMICOIIDUR MATERIAL FROM A MELT IN A CRUCIBLE Filed Dec. 5, 1963 Fig.4

F|g.3 Fl 9 6 31 Dr l Dr Fig.7 u 33 T was in I=f(l) ml v ggv lm UnitedStates Patent 3 Claims. (31. 23-473 My invention relates to the methodof pulling rodshaped crystals of semiconductor material from a moltenmass of the material contained in a crucible, and has for its mainobject to reliably provide for a constant cross section of thecrystalline rod being produced.

When producing crystals by pulling them out of a melt in a crucible, itis difficult to accurately determine the diameter of the resultingrod-shaped crystals and, above all, to maintain the diameter at adesired constant value during the entire course of the pullingoperation. While it is known to regulate the diameter of therecrystallizing rod during crucible-free (floating) zone melting byinductively sensing the diameter with the aid of the inductionheatercoil employed for producing the molten zone or by means of an additionalmeasuring coil, or also with the aid of optical scanning devices, theapplication of such diameter-responsive sensing means to the pulling ofcrystals from a melt in a crucible is not readily possible because it isextremely difficult, if at all feasible, to place the sensing orscanning devices sufiiciently close to the liquid-solid boundry of thesolidifying material, This difiiculty is to a great extent due to thefact that the crucible, prior to melting, supplied with chunks of solidmaterial so that the sensing devices could be placed close to themeasuring location only atter the solid pieces have been melted, butthis would require an extremely accurate and careful observation andcontrol of the sensor adjusting motion. This, however, is greatlyimpeded by the crucible rim and often also by heat shielding and otherauxiliary means that prevent of obscure direct observation.

Another difiiculty of regulating the diameter of the growing rod-shapedcrystal for constancy is due .to the fact that the level of the melt inthe crucible is not constant and that level fluctuations cannot besufficiently avoided even with the aid of .a regulating device.

It is an object of my invention to devise a method of pulling rod-shapedcrystals of semiconductor material from a melt in a crucible that avoidsthe above-mentioned short-comings and difficulties and affords byrelatively simple means an accurate control of the diameter or crosssection of the rod being pulled from the melt so as to maintain the roddiameter at a desired constant value.

According to my invention, the adjustment and maintenance of a given roddiameter during the pulling operation is effected by continuouslydetermining the weight (W) and the length (L) of the rod being pulledand hence the weight-to-length differential quotient (dW/dL) thatcorresponds to the datum value of the rod diam- -tected with the aid oftransducers that furnish respective measuring voltages, and these arecompared with each other in an electric network that furnishes an errorvoltage for controlling the crystal pulling operation.

The continuous determination and regulation of the rod diameter byascertaining the rod weight (W) in dependence upon the pulling length(L) affords performing all necessary measuring operations With the aidof sensing means that need not be located in the immediate vicinity ofthe liquid-solid phase boundary, thus eliminating the fundamentaldifficulties of the above-mentioned other methods. When operating,according to another feature of my invention, at a constant crystalpulling speed, the Weight of the rod being pulled increases inproportion to time; consequently it is then only necessary to controlthe temperature of the melt in dependence upon the continuously measuredweight of the rod being pulled. However, the control of the pullingoperation according .to the invent-ion can be effected also by varyingthe pulling speed.

Particularly in cases where the resulting semiconductor crystals are tohave uniform doping, it is preferable to perform the method at constantcrystal pulling speed and to vary the temperature of the melt. In thiscase, the preferably electric measuring magnitude or signal voltageresulting from the change in weight of the rod is employed by means ofsuitable transmitting devices for controlling the temperature of themelt in the crucible. For maintaining a constant pulling speed, asynchronous electric motor may be employed, or also a motor equippedwith a centrifugal-type speed regulator so as to operate the pullingdevice at a constant driving speed.

When performing the method of the invention while manitaining thetemperature of the melt at a constant value, the crystal pulling speedis to be varied in an analogous manner under control by the measuringmagnitude or signal voltage resulting from the changes in weight of thecrystal.

According to another feature of my invention it is preferable to sensethe crystal weight with the aid of resistance strain gauges. It is ofadvantage to employ several strain-gauge strips in order to eliminateerrors due to bending and deformation perpendicularly to the directionof the strain to be sensed. By connecting several strain-gauge strips inseries, a multiple of the slight resistance changes that occur as aresult of changes in crystal weight, can be obtained. The accuracy foundattainable with such a device is particularly satisfactory. For example,with a crystal diameter of 28.0 mm, a change in diameter of 0.25 mm. canstill be ascertained in this manner.

Applicable in lieu of strain gauges are any other suitable deformationorforce-responsive sensors and transducers, for example piezoelectrictransducers. The in crease in crystal weight can also be determinedindirectly by continuously measuring the reduction in total weight ofthe melt-containing crucible.

When operating with resistance strain gauges or other resistivetransducers in the above-described manner, the change in electricresistance resulting from the change in crystal Weight is preferablyutilized by connecting the weight-sensing resistor in a bridge networkor other comparator network together with a continuously varyingreference resistor so that the corresponding voltage drops are opposedto each other and jointly furnish a difference voltage which, since onlyvery small values of this voltage are effective, is indicative of theabove-mentioned differential quotient: The occurring differential enrorvoltage is then employed, preferably upon amplification in an electronicamplifier, for controlling the temperature of the melt in the crucibleor for controlling the crystal pulling speed.

Analogously, when operating with a constant tempera- Patented July 5,1966 '21 for the crucible 1.

ture of the melt, the control of the crystal pulling operation accordingto the invention can be effected by varying the pulling speed undercontrol by the differential error voltage resulting from the comparisonof the weightresponsive current or voltage with a reference current orvoltage varying in dependence upon the length of the crystal beingpulled and hence in accordance with the pulling travel.

The foregoing and other objects, advantages and features of myinvention, said features being set forth with particularly in the claimsannexed hereto, will be apparent from the following description ofembodiments illustrated by way of example in the accompanying drawing,in which:

FIG. 1 shows schematically and in section a crystal pulling apparatusoperating in accordance with the invention.

FIG. 2 is a horizontal cross section through a tubular 1 pullingstructure and strain gauges that form part of the apparatus according toFIG. 1.

FIG. 3 is a schematic diagram of a temperature regulating circuitapplicable in conjunction with the apparatus according to FIGS. 1 and 2.

FIG. 4 shows schematically an electric drive for the pulling structureof apparatus otherwise as shown in FIG. 1, but in con-junction with anelectric control system for operation at constant temperature of themelt and variable crystal pulling speed.

FIGS. 5, 6 and 7 show three different modifications of regulatingcircuits applicable in lieu of the regulating circuit according to FIG.3, in conjunction with apparatus as shown in FIG. 1.

As shown in FIG. 1, an evacuated processing vessel 10 'contains acrucible 1 with molten semiconductor material 2 such as silicon,germanium, indium antimonide or any other semiconductor substancesuitable for the production of mono-crystals in the above-describedmanner. The top portion of the processing vessel 10 has a central neck 5with a cylindrical seal 6 traversed by a pulling structure 9 in form ofa cylindrical and tubular rod. The rod-shaped crystal is fastened bymeans of a clamping device 4 to the bottom portion 8 of the tubularstructure 9. As a rule, and as shown, only a thin see-d crystal isclamped in the holder 4 and, when commencing the pulling process, ispartly immersed in the melt 2, to be thereafter withdrawn from the meltwith the result that semiconductor material will crystallize to the seedand thereby produce a rod whose diameter depends upon the temperature ofthe melt as well as upon the pulling speed. During pulling operation,the increase in weight of the crystal 3 is continuously sensed bystrain-gauge strips 7 which are cemented to the wall of the thin-walledend portion 8 of the pulling structure 9. The electric leads 11 and 12of the strain gauges 7 extend upwardly through the interior of thetubular structure 9 to the outside of the processing vessel, the bottomof the tubular structure 9 being hermetically sealed for preserving thevacuum in the vessel during pulling operation, it being understood thatit is preferable to have the vessel continuously connected to a vacuumpump and also to provide for opening the vessel before commencing andafter terminating the crystal pulling operation. The gauge strips areprotected in tube portion 8 from being heated by radiation from the melt2. For this purpose, cooled heat-shielding members (not shown) may beadded if desired, or a coolant may be supplied to the gauge stripsthrotigh the interior of the tubular pulling structure 9.

schematically shown in FIG. 1 is a resistance heater The heater 21 isconnected in series with an adjustable rhe-ostat 22 and through anamplifier 23 to the output terminals 33, 34 of the bridge network shownin FIG. 3. This circuit diagram relates to operation at constant pullingspeed and hence uniform upward travel of the tubular pulling structure 9during steady-state operation. In FIG. 3 the variable resistance of thestrain gauges 7 is denoted by R This resistance, varying in dependenceupon the increasing Weight of the crystal, is compared in the bridgenetwork with a resistance R1 which varies continuously in proportion tothe pulling speed and consequently at a constant rate of change.

provide the signal voltage which, upon amplification in amplifier 23(FIG. 1), serves for controlling the temperature of the melt. Theregulation tends to vary the temperature of the melt 2 in the senserequired for reducing the differential error voltage. As a result, thediameter of the crystal rod is maintained at the desired constant value.

When operating with a constant temperatur of the melt 2, thedifferential signalvoltage issuing from the above-described bridgecircuit is used for correspondingly controlling the pulling speed. Oneway of doing this is schematically shown in FIG. 4. During operation,the tubular pulling structure 9 is displaced upwardly by pinch rollers39 and 40 driven from the armature 41 of a motor which has a main fieldwinding 42 and a speed-controlling field winding 43. Winding 43 isexcited through a control rheostat 44 whose tap is displaced by asolenoid 45 in the output diagonal of a direct-current bridge networkessentially corresponding to the one described above with reference toFIG. 3.

Another way of practicing the invention, relating to operation atconstant crystal pulling speed, is to use the weight-responsivemeasuring current for charging a capacitor, periodically discharging thecapacitor, and comparing the resulting capacitor discharge voltage witha controlled reference voltage having a constant rate of change inaccordance with the constant pulling speed. The resulting differentialerror voltage is then available for controlling the temperature of themelt. The modified regulating network shown in FIG. 5, operates in thismanner. a

It will be understood that terminals 11 and 12 according to FIG. 5 areto be connected to the leads 11 and 12 according to FIG. 1, the outputterminals 33 and 34 according to FIG. 5 being identical with thosedesignated by 33 and 34 in FIG. ll. In the network of FIG. 5, acapacitor C is continuously charged by current whose magnitude iscontrolled by the strain gauges 7 and consequently is determined by thevarying weight of the crystal r-od being pulled at constant speed. Thecapacitor C is periodically discharged by actuation of a switch S,preferably an electronic switch operating in given intervals of time.The capacitor discharge produces a voltage drop U along a resistor R4.This voltage drop is compared with a controllaible voltage U which,aside from the internal resistance of the voltage source, is equal tothe voltage drop U which it produces across a resistor R6. The voltagedifference AU: U U U U appears across a resistor R5 and between theoutput terminals 33, 34 and thus is employed for regulating the crystalpulling operation for constancy of the rod diameter.

According to the embodiment of the regulating circuit shown in FIG. 6,the measuring current, varying in dependence upon the crystal weight, ispassed through an inductance coil D and the reactive voltage U occurringalong the coil is impressed between the output terminals 33 and 34 forcontrolling the crystal pulling operation.

The regulating network according ,to FIG. 7 corre- This variation may beeffected, for example, by providing the resistor R1 with a slide contactas shown sponds in principle to that described above with reference toFIG. 5, except that the reference voltage U is not produced 'with theaid of voltage from an auxiliary generator but is derived from thecurrent 1: (L) that is a function of the rod length and hencecontrolledby the abovementioned resistor R1 (FIG. 4). voltage U is compared withthe voltage drop U; of a resistor R7 connected between the terminals 11and 12 and consequently dependent upon the weight-responsive current 1:(G). The voltage diiference appears at a resistor R9 and is impressedacross the terminals 33 and 34. This difference voltage is employed forcontrolling the pulling operation in the above-described manner.

To those skilled in the art it will be obvious upon a study of thisdisclosure that my invention permits of various other modifications withrespect to processing equipment and circuitry and consequently may begiven embodiments other than particularly illustrated and describedherein, without departing from the essential features of my inventionand within the scope of the claims annexed hereto.

I claim:

1. Apparatus for pulling rod-shaped crystals of semiconductor materialfrom a melt in a crucible, comprising a processing vessel;

a crucible containing the melt positioned in said processing vessel;

heater means in operative proximity with said crucible for heating themelt at a substantially constant temperature;

crystal pulling means mounted in said vessel above said crucible forsubstantially vertical movement, said crystal pulling means comprising asubstantially vertically displaceable member, driving means in operativeproximity with said member for moving said member in a substantiallyvertical direction and crystal holder means on said member for seedingand pulling a crystal rod upwardly out of the melt when said member ismoved upwardly;

strain gage means amxed to the member of said crystal pulling means forproviding an electrical signal having a magnitude indicative of theweight of said crystal rod;

circuit means including variable resistance means having aresistance-varying arm mechanically coupled to the driving means of saidcrystal pulling means for providing an electrical signal having amagnitude indicative of the length of said crystal rod;

electrical comparator means including said strain gage means and saidcircuit means for comparing the electrical signals corresponding to theweight and the length of the crystal rod to produce an electric controlsignal proportional to the differential quotient of the rate of changeof weight to the rate of change of length of said crystal rod; and

control means connected to said electrical comparator means and coupledto the driving means of said crystal pulling means for controlling thepulling speed by the electric control signal to maintain thedifierential quotient constant thereby to maintain the diameter of therod-shaped crystal constant.

2. Apparatus for pulling rod-shaped crystals of semiconductor materialfrom a melt in a cruicible, comprising a processing vessel;

a crucible containing the melt positioned in said processing vessel;

heater means in operative proximity with said crucible for heating themelt at a substantially constant temperature; crystal pulling meansmounted in said vessel above said This length-responsive crucible forsubstantially vertical movement, said crystal pulling means comprising asubstantially vertically displaceable member, driving means in operativeproximity with said member for moving said member in a substantiallyvertical direction and crystal holder means on said member for seedingand pulling a crystal rod upwardly out of the melt when said member ismoved upwardly;

piezoelectric force gage means aflixed to the member of said crystalpulling means for providing an electrical signal having a magnitudeindicative of the weight of said crystal rod;

circuit means including variable resistance means havin g aresistance-varying arm mechanically coupled to the driving means of saidcrystal pulling means for providing an electrical signal having amagnitude indicative of the length of said crystal rod;

electrical comparator means including said piezoelectric force gagemeans and said circuit means for comparing the electrical signalscorresponding to the weight and the length of the crystal rod to producean electric control signal proportional to the diiferential quotient ofthe rate of change of weight to the rate of change of length of saidcrystal rod; and

control means connected to said electrical comparator means and coupledto 'the driving means of said crystal pulling means 'for controlling thepulling speed by the electric control signal to maintain thedifferential quotient constant thereby to maintain the diameter of therod-shaped crystal constant.

3. Apparatus for pulling rod-shaped crystals of semiconductor materialfrom a melt in a crucible, comprising crystal pulling means mounted insaid vessel above said crucible for substantially vertical movement,said crystal pulling means comprising a substantially verticallydisplaceable elongated tube extending from a sealed lower end in saidprocessing vessel through said vessel to an open upper end outside saidvessel, driving means in operative proximity with said tube for movingsaid tube in a substantially vertical direction and crystal holder meanson said tube for seeding and pulling :a crystal rod upwardly out of themelt when said tube is moved upwardly;

resistance strain gage means afiixed to the inside of the tube of saidcrystal pulling means near the lower end thereof for providing anelectrical signal having a magnitude indicative of the weight of saidcrystal rod;

electrical leads connected to said resistance strain gage means andextending upwardly through the tube of said crystal pulling means andthrough the upper end of said tube to outside said vessel;

circuit means including variable resistance means having aresistance-varying arm mechanically coupled to the driving means of saidcrystal pulling means for providing an electrical signal having amagnitude indicative of the length of said crystal rod;

electrical comparator means including said resistance strain gage meansand said circuit means for comparing the electrical signalscorresponding to the weight and the length of the crystal rod to producean electric control signal proportional to the diiferentional quotientof the rate of change of weight to the rate of change of length of saidcrystal rod; and

control means connected to said electrical comparator means and coupledto the driving means of said crystal pulling means for controlling thepulling speed by the electric control signal to maintain the 7differential quotient constant thereby to maintain 2,992,311 7/ 1961Keller 23 -301 the diameter of the rod-shaped crystal constant.3,046,379 7/ 1962 Keller et a1 23-301 References Cited by the ExaminerDAVID L. RECK, Primary Examiner.

UNITED STATES PATENTS 5 HYLAND BIZOT, Examiner.

2,908,004 10/ 1959 Levinson 1 N. F, MARKVA, Assistant Examiner.

2,913,561 111/1959 Rummel et a1. 1481.6

1. APPARATUS FOR PULLING ROD-SHAPED CRYSTALS OF SEMICONDUCTOR MATERIALFROM A MELT IN A CRUCIBLE, COMPRISING A PROCESSING VESSEL; A CRUCIBLECONTAINING THE MELT POSITIONED IN SAID PROCESSING VESSEL; HEATER MEANSIN OPERATIVE PROXIMITY WITH SAID CRUCIBLE FOR HEATING THE MELT AT ASUBSTANTIALLY CONSTANT TEMPERATURE; CRYSTAL PULLING MEANS MOUNTED INSAID VESSEL ABOUT SAID CRUCIBLE FOR SUBSTANTIALLY VERTIICAL MOVEMENT,SAID CRYSTAL PULLING MEANS COMPRISING A SUBSTANTIALLY VERTICALLYDISPLACABLE MEMBER, DRIVING MEANS IN OPERATIVE PROXIMITY WITH SAIDMEMBER FOR MOVING SAID MEMBER IN A SUBSTANTIALLY VERTICAL DIRECTION ANDCRYSTAL HOLDER MEANS ON SAID MEMBER FOR SEEDING AND PULLING A CRYSTALROD UPWARDLY OUT OF THE MELT WHEN SAID MEMBER IS MOVED UPWARDLY; STRAINGAGE MEANS AFFIXED TO THE MEMBER OF SAID CRYSTAL PULLING MEANS FORPROVIDING AN ELECTRIICAL SIGNAL HAVING A MAGNITUDE INDICATIVE OF THEWEIGHT OF SAID CRYSTAL ROD; CIRCUIT MEANS INCLUDING VARIABLE RESISTANCEMEANS HAVING A RESISTANCE-VARYING ARM MECHANICALLY COUPLED TO THEDRIVING MEANS OF SAID CRYSTAL PULLING MEANS FOR PROVIDING AN ELECTRICALSIGNAL HAVING A MAGNITUDE INDICATIVE OF THE LENGTH OF SAID CRYSTAL ROD;ELECTRICAL COMPARATOR MEANS INCLUDING SAID STRAIN GAGE MEANS AND SAIDCIRCUIT MEANS FOR COMPARING THE ELECTRICAL SIGNALS CORRESPONDING TO THEWEIGHT AND THE LENGTH OF THE CRYSTAL ROD TO PRODUCE AN ELECTRIC CONTROLSIGNAL PROPORTIONAL TO THE DIFFERENTIAL QUOTIENT OF THE RATE OF CHANGEOF WEIGHT TO THE RATE OF CHANGE OF LENGTH OF SAID CRYSTAL ROD; ANDCONTROL MEANS CONNECTED TO SAID ELECTRICAL COMPARATOR MEANS AND COUPLEDTO THE DRIVING MEANS OF SAID CRYSTAL PULLING MEANS FOR CONTROLLING THEPULLING SPEED BY THE ELECTRIC CONTROL SIGNAL TO MAINTAIN THEDIFFERENTIAL QUOTIENT CONSTANT THEREBY TO MAINTAIN THE DIAMETER OF THEROD-SHAPED CRYSTAL CONSTANT.